The present invention relates to a method and device for implementing a differential lock function for a vehicle.
The production or implementation of a differential lock function, especially the implementation of an interaxle lock function, by actively decelerating wheels having a drive slip that is too high, is known from the related art. In known methods heretofore, an interaxle lock function is usually produced or implemented by using so-called individual control systems. In this context, the individual wheel control system simultaneously assumes the locking function in the transverse direction, and in the longitudinal direction.
German Published Patent Application No. 34 21 776 describes a vehicle having all-wheel drive. In this vehicle, not only are the wheels of one axle connected to the drive shaft, but also the drive shafts are connected to the driving motor via a differential gear. The object of German Published Patent Application No. 34 21 776 is to provide an electronic alternative for the differential locks. The electronic differential locking is implemented by supplying braking pressure to the individual wheel brakes in response to the occurrence of drive slip. That is, one or more wheels are decelerated when this wheel or these wheels undergoes or undergo wheelslip in comparison to the other wheels. When the last wheel slips as well, then the engine torque is reduced. In other words, all of the wheels are adjusted to an optimum drive slip, and therefore allow good lateral grip.
With regard to the implementation of the differential lock function, one can only deduce the supply of braking pressure from German Published Patent Application No. 34 21 776. The use or provision of a specific requirement regarding the level of the braking pressure to be supplied cannot be gathered from this document. Consequently, one can also not infer from it that, within the framework of implementing the differential lock, a braking moment is set in accordance with a preselected setpoint value. Therefore, the actions carried out in this manner do not ensure that the speeds of the driven vehicle axles approach each other in a desired manner.
Therefore, the object of the present invention consists in improving existing methods and devices for implementing a differential lock function.
The method of the present invention is a method for implementing a differential lock function for a vehicle. In response to incipient slippage of at least one driven wheel the method of the present invention implements the function of a differential lock, using actions carried out independently of the driver, on at least one arrangement for controlling the wheel torque. The implemented differential lock function should preferably be a differential lock acting between the front axle and the rear axle of the vehicle. At least one setpoint value for a wheel torque to be set is selected for carrying out the actions executed independently of the driver.
The vehicle is advantageously a vehicle having all-wheel drive. Therefore, the differential lock function is an interaxle lock acting between the front axle and the rear axle of the vehicle. However, this should not represent a limitation. The present invention""s method for implementing an interaxle lock function, which is represented for an all-wheel drive vehicle in the exemplary embodiment, can also be appropriately modified for use in a vehicle having a single driven axle, in order to implement an axle differential locking function. However, this use shall not be discussed in any more detail within the framework of the present application.
The selection of the at least one setpoint valuexe2x80x94there are two driven axles in a vehicle having all-wheel drive, which is why two setpoint values are selectedxe2x80x94allows the speeds of the driven vehicle axles to approach each other. This measure assures optimum traction for the vehicle.
The arrangement for controlling the wheel torque is advantageously a brake actuator, which is assigned to a wheel of the vehicle, and is a part of a braking system that can generate braking torques at individual vehicle wheels, independently of the driver. On the other hand, the braking system can be a hydraulic, electrohydraulic, pneumatic, electropneumatic, or electromechanical braking system. A controllable mechanical locking device or a controllable clutch can be used as an alternative to the brake actuator.
Such a controllable mechanical locking device or controllable mechanical clutch could be used to bypass an open, center differential, or to directly couple the second drive axle to the main drive axle, as would be possible in the case of a front-wheel drive vehicle having a coupleable rear-wheel drive. In this case, the coupling torque to be transmitted, which results from the equation |MBrSymLSVAxe2x88x92MBrSymLSHA|, would be used as a setpoint value. This setpoint value is input via a torque interface to the secondary controller, as a setpoint value for the coupling torque.
If the arrangement for controlling the wheel torque is a brake actuator, then a setpoint value for a wheel-brake torque is advantageously selected as a setpoint value. In the case of a vehicle, which has all-wheel drive, and consequently has two driven axles, a setpoint value is selected for each of the two axles.
A setpoint value for the Cardanic speed of an axle is ascertained as a function of first wheel speed variables that describe the free-rolling wheel speeds, and as a function of a setpoint value for the drive slip. An actual value for the Cardanic speed of the same axle is ascertained as a function of second wheel speed variables, which describe the wheel speeds ascertained with the aid of wheel speed sensors. A deviation variable is determined as a function of this setpoint value and this actual value. Using a controlling arrangement, in particular a PI controller, this deviation variable is converted into the setpoint value for a wheel torque to be set.
The setpoint value for the Cardanic speed is selected in such a manner, that the differential lock function is activated as a function of time, prior to controlling the engine torque. To this end, the setpoint value for the Cardanic speed is especially ascertained as a function of a speed variable, which describes the vehicle speed. This procedure is based on the following: Measures are initially taken in the starting range, in order to improve the traction by locking action. As long as this potential is not exhausted, the engine torque is only reduced slightly. The drive torque is only reduced sharply, when all four wheels have too much drive slip, or the vehicle becomes unstable. This is accomplished by interventions in the engine-torque control.
Effects in the second wheel speed variables, which originate from reciprocal oscillations of the wheel speeds at the respective axle, are considered in the determination of the deviation variable.
It is advantageous, when the controlling arrangement has a proportional component and an integral component. The integration gain for the integration component is ascertained as a function of the value of the differentiated deviation variable. The integral component is reduced quickly, when at least one wheelslip variable for the respective axle exceeds a predetermined value. The wheelslip is monitored for the following reason: Because of the active build-up of braking pressure at the drive wheel, it can never be ruled out that this wheel is being overly decelerated. This results in brake slip at the overly decelerated wheel, which is to be immediately eliminated by reducing the pressure.
In the case of all-wheel drive vehicles, optimum traction can only be achieved when all of the driving wheels are rotating at speeds that are as uniform as possible. In particular, no differential speed, or as small a differential speed as possible, should occur between front axle (VA) and rear axle (HA), in order that the engine torque can be optimally transmitted through the wheels, to the ground. This requirement can be met with the aid of the method and device according to the present invention.
The function of a differential lock in the longitudinal direction, i.e. between the front axle and the rear axle of the vehicle, should be realized by an independent control loop. In this manner, the resulting interaxle locking effect can be controlled individually. This should be achieved by a wheel speed controller based on setpoint slip values.
For this purpose, the vehicle is advantageously equipped with a slip control device, as is known, e.g. from the article xe2x80x9cFDRxe2x80x94Die Fahrdynamikregelung von Boschxe2x80x9d (xe2x80x9cESPxe2x80x94The Electronic Stability Program of Boschxe2x80x9d) published in the Automobiltechnischen Zeitschrift (Automobile Technology Magazine) (ATZ) 96, 1994, issue 11, pages 674 through 689. In addition, at least one vehicle motion variable can be controlled by this device. This vehicle motion variable is the yaw velocity of the vehicle. To control the yaw velocity of the vehicle, the measured yaw velocity is compared to a setpoint value for the yaw velocity, and the yaw velocity deviation is ascertained, as a function of which wheel-specific braking actions and/or engine interventions are carried out independently of the driver. To this end, setpoint slip changes are determined as a function of the yaw velocity deviation, and the setpoint slip values for the individual wheels are ascertained from the setpoint slip changes. These setpoint slip values are supplied to controllers, which are subordinate to the yaw velocity controller, and by way of which the brake slip or the drive slip can be controlled. In order to set a wheel speed, the setpoint slip value is specially converted to a setpoint wheel speed in the subordinate controller that controls the drive slip. Above all, the wheel-specific braking actions that are independent of the driver apply a yaw torque to the vehicle, by way of which the actual yaw velocity of the vehicle approaches the setpoint value for the yaw velocity.
In the meantime, the above-described operating dynamics control system is now widely referred to as ESP (Electronic Stability Program). As a result, the content of the publication xe2x80x9cESPxe2x80x94The Electronic Stability Program of Boschxe2x80x9d shall be a part of this application.
The above reference to the device for controlling operating dynamics should not represent a limitation. Of course, the vehicle can also be equipped with a device for controlling drive slip, or with another device for controlling slip. However, the device for controlling slip executes wheel-specific braking actions, so-called active braking actions, independently of the driver, since these implement the method of the present invention.
The interaxle lock controller (L-controller) of the present invention is advantageously realized in such a manner, that it can be integrated into an existing electronic stability program for any all-wheel drive vehicle. The following measures ensure that the interaxle lock controller can be easily integrated: Only input variables based on physical quantities are used for the interaxle lock controller. The two torques MBrSymLSVA and MBrSymLSHA are used as output variables for the case, in which the arrangement for controlling the wheel torque are brake actuators. However, the coupling torque |MBrSymLSVAxe2x80x94MBrSymLSHA| is used in the case, in which the arrangement for controlling the wheel torque is a controllable locking device or a controllable clutch. Any actuator functioning on the basis of torque, whether it is a controllable coupling or a controllable locking device, can be rapidly coupled to the interaxle lock controller. If this is not possible, a special interface is provided. Such an interface may be a characteristics map for the coupling-actuator control current as a function of the coupling torque.
Furthermore, the interaxle lock controller of the present invention is realized in such a manner, that it functions in harmony with an engine torque control system (AMR), if necessary. In addition, it satisfies offroad requirements, i.e. it can be used in vehicles traveling on a rough road surface, or used for traveling offroad. The following measures are taken for this: The maximum interaxle lock torque (MBrSymIMax) becomes larger with increasing control period. Above certain, high engine torques, the reductions in engine torque becoming increasingly weak. In difficult offroad terrain, there is almost no more engine torque reduction. The engine torque is more distinctly reduced on low coefficients of friction, which allows one to start smoothly from rest.
Additional advantages include:
The Cardanic speeds of each axle are synchronized. For dynamic reasons (vibration damping), it is advantageous when each axle can be controlled individually. This is in close conjunction with the specific embodiment as a brake-interaxle lock.
The interaxle lock torque is built up using a PI controller, since a system deviation exists in the case in which the interaxle lock has to increase the locking torque. Whereas the interaxle lock torque is reduced with the aid of a control system. In this case, there is no deviation that can be evaluated.
Geometric slip, which can possibly occur from cornering, is calculated. Assuming that cornering and the associated geometric slip could possibly occur, this measure ensures that no actions are carried out that are not really necessary. For example, the so-called geometric slip occurs during cornering, since the front wheels are turning at a smaller radius than the corresponding rear wheels. In the absence of compensating measures, a system deviation would occur which would activate the interaxle lock in response to tight cornering. This effect is calculated by taking the kinematic interrelationships into account. It is already taken into consideration during the determination of the free-rolling wheel speeds. The dead zone of the interaxle lock controller would have to be expanded, if the geometric slip were not compensated for.
A setpoint torque is output axle-specifically, as an actuating variable, to a secondary actuator. The actuator can be a braking system, which can actively build up pressure, i.e. independently of the driver. A controllable, mechanical locking device or a controllable mechanical coupling, which acts between the front axle and the rear axle, can also be considered as an actuator. However, the actuator in the latter case must be supplied an actuating variable, which represents the difference to be set in the setpoint torque between the front axle and the rear axle. The coupling torque |MBrSymLSVAxe2x80x94MBrSymLSHA| is suited for this.
The dynamics of the interaxle lock controller can be adjusted independently of the axle differential locks. This is achieved by using an independent controller having its own set of parameters, in order to implement the interaxle lock function. At the same time, the working point can be individually adjusted in a manner enabling a harmonious interaction with an engine-torque control system.
When needed, stall protection for vehicles having a manual transmission prevents the engine from stalling, which can possibly be triggered by activities of the interaxle lock controller. The stall protection relates to the following issue: The actions of the present invention on the arrangement for controlling the wheel torque can decelerate the wheels so sharply, that it is possible for the vehicle engine to stop. Appropriate monitoring is used to prevent this.
The stall protection is implemented as follows: The engine speed is monitored. If it falls below a specific minimum value, then the interaxle lock torque is sharply reduced in response thereto, until the engine speed has exceeded this minimum value again. The minimum value for the engine speed can be selected as a function of the interaxle lock torque itself, using a characteristic curve. It is also ensured that the interaxle lock torque is limited to a percentage of the instantaneously available, Cardanic drive torque.